#include "devices/intq.h"
#include <debug.h>
#include "threads/thread.h"

static int next(int pos);
static void wait(struct intq *q, struct thread **waiter);
static void signal(struct intq *q, struct thread **waiter);

/* Initializes interrupt queue Q. */
void intq_init(struct intq *q)
{
  lock_init(&q->lock);
  q->not_full = q->not_empty = NULL;
  q->head = q->tail = 0;
}

/* Returns true if Q is empty, false otherwise. */
bool intq_empty(const struct intq *q)
{
  ASSERT(intr_get_level() == INTR_OFF);
  return q->head == q->tail;
}

/* Returns true if Q is full, false otherwise. */
bool intq_full(const struct intq *q)
{
  ASSERT(intr_get_level() == INTR_OFF);
  return next(q->head) == q->tail;
}

/* Removes a byte from Q and returns it.
   If Q is empty, sleeps until a byte is added.
   When called from an interrupt handler, Q must not be empty. */
uint8_t
intq_getc(struct intq *q)
{
  uint8_t byte;

  ASSERT(intr_get_level() == INTR_OFF);
  while (intq_empty(q))
  {
    ASSERT(!intr_context());
    lock_acquire(&q->lock);
    wait(q, &q->not_empty);
    lock_release(&q->lock);
  }

  byte = q->buf[q->tail];
  q->tail = next(q->tail);
  signal(q, &q->not_full);
  return byte;
}

/* Adds BYTE to the end of Q.
   If Q is full, sleeps until a byte is removed.
   When called from an interrupt handler, Q must not be full. */
void intq_putc(struct intq *q, uint8_t byte)
{
  ASSERT(intr_get_level() == INTR_OFF);
  while (intq_full(q))
  {
    ASSERT(!intr_context());
    ASSERT(!lock_held_by_current_thread(&q->lock));

    lock_acquire(&q->lock);
    wait(q, &q->not_full);
    lock_release(&q->lock);
  }

  q->buf[q->head] = byte;
  q->head = next(q->head);
  signal(q, &q->not_empty);
}

/* Returns the position after POS within an intq. */
static int
next(int pos)
{
  return (pos + 1) % INTQ_BUFSIZE;
}

/* WAITER must be the address of Q's not_empty or not_full
   member.  Waits until the given condition is true. */
static void
wait(struct intq *q UNUSED, struct thread **waiter)
{
  ASSERT(!intr_context());
  ASSERT(intr_get_level() == INTR_OFF);
  ASSERT((waiter == &q->not_empty && intq_empty(q)) || (waiter == &q->not_full && intq_full(q)));

  *waiter = thread_current();
  thread_block();
}

/* WAITER must be the address of Q's not_empty or not_full
   member, and the associated condition must be true.  If a
   thread is waiting for the condition, wakes it up and resets
   the waiting thread. */
static void
signal(struct intq *q UNUSED, struct thread **waiter)
{
  ASSERT(intr_get_level() == INTR_OFF);
  ASSERT((waiter == &q->not_empty && !intq_empty(q)) || (waiter == &q->not_full && !intq_full(q)));

  if (*waiter != NULL)
  {
    thread_unblock(*waiter);
    *waiter = NULL;
  }
}
